When EGPRS was standardized, legacy GPRS mobile stations (MSs) were required to decode the Uplink State Flag (USF) of EGPRS radio blocks. EGPRS radio blocks can be modulated utilizing either Gaussian Minimum Shift Keying (GMSK) as in Modulation and Coding Schemes MCS-1 to MCS-4, or Eight-Phase Shift Keying (8PSK) as in Modulation and Coding Schemes MCS-5 to MCS-9. GPRS MSs cannot receive 8PSK-modulated EGPRS radio blocks, but a solution was found enabling GPRS MSs to receive GMSK-modulated EGPRS radio blocks. The solution was to encode and interleave the USF of the GMSK-modulated EGPRS radio blocks exactly the same way as one of the GPRS coding schemes, namely CS-4. The GPRS MS was led to believe that a CS-4 radio block was received by putting stealing bits in the GMSK-modulated EGPRS radio blocks in the same positions as in the legacy GPRS radio blocks and setting these stealing flags to the codeword for CS-4. Consequently, unless the radio conditions were too bad, the GPRS MS could successfully decode the USF believing the block was a CS-4 radio block. Subsequently, the GPRS MS would attempt to decode the rest of the EGPRS radio block as a CS-4 block and fail (detected by a CRC failure). However, this solution still allows for satisfying the primary objective of using the USF, received within the context of an MCS-5 to MCS-9 encoded EGPRS radio block, to schedule uplink transmissions for a legacy GPRS MS.
EGPRS MSs also read the legacy stealing bits, but for the EGPRS MS, the CS-4 stealing bit code word indicates that an EGPRS radio block has been sent using one of the Modulation and Coding Schemes MCS-1 to MCS-4. Consequently, an EGPRS MS can successfully decode the USF since the USF is placed in the right position (same as for CS-4). In order to determine which of MCS-1 to MCS-4 has been used, the EGPRS MS decodes the RLC/MAC header and looks at the Coding and Puncturing Scheme (CPS) field, and decodes the rest of the radio block. If, the radio block actually was a CS-4 radio block, this latter part will fail (detected by a CRC failure during RLC/MAC header decoding). However, this solution also still allows for satisfying the primary objective of using the USF, received within the context of a CS-1 to MS-4 encoded GPRS radio block, to schedule uplink transmissions for an EGPRS MS.
EGPRS MSs must also be able to decode the USF of GPRS CS-1 to CS-3 radio blocks. Therefore, they must be able to decode the stealing bit code words of these coding schemes as well.
Thus for an EGPRS MS, the MS reads the stealing bits and if it detects the CS-4 stealing bit code word it causes the EGPRS MS to think that an MCS-1 to MCS-4 block has been received. The EGPRS MS can successfully decode the USF if the CS-4 stealing bit code word is used since for MCS-1 to MCS-4 the USF is carried in the same way as for CS-4 blocks. The EGPRS MS then attempts to decode the RLC/MAC header (the CPS field in particular) to see which of the MCS-1 to MCS-4 blocks has been received. If an MCS-1 to MCS-4 block was really sent, then the EGPRS MS successfully decodes the RLC/MAC header. If a CS-4 block was really sent, then the EGPRS MS fails to successfully decode the RLC/MAC header. The EGPRS MS is also able to decode the CS-1 to CS-3 stealing bit code words and therefore read the USF sent along with CS-1 to CS-3 blocks. However, if these USFs are detected, the EGPRS MS shall not attempt to decode the RLC/MAC header to try and verify the presence of a MCS-1 to MCS-3 block (i.e., only for the case of a CS-4 stealing bit code word will the EGPRS MS be able to read the USF and potentially receive an MCS-1 to MCS-4 block).
For a GPRS MS, if a CS-4 stealing bit code word is read, it causes the GPRS MS to think that a CS-4 block has been received. The GPRS MS attempts to decode the RLC/MAC header as if a CS-4 block has been received. If a CS-4 block was sent, then the GPRS MS successfully decodes the RLC/MAC header. If an MCS-1 to MCS-4 block was really sent, then the GPRS MS fails to successfully decode the RLC/MAC header.
FIG. 1 illustrates a radio block 10 coded using the conventional GPRS coding scheme CS-1. In EGPRS, the RLC/MAC control messages are sent using the GPRS coding scheme CS-1. This is for legacy reasons, to allow GPRS MSs to read the same message in case the content is a distribution message. In the current specification for EGPRS, MSs read the stealing bits first, and then decode the USF according to the value of the stealing bits (SBs). For a CS-1 encoded downlink block, the USF is encoded together with the payload as shown in block 11 and can only be obtained after convolutional decoding of the entire radio block.
In release 7 of the 3GPP standard, a new type of radio block—Reduced Transmit Time Interval (RTTI)—is defined. RTTI radio blocks are transmitted in 10 ms over two timeslots instead of in 20 ms over one timeslot. A detailed description of RTTI is found in the 3GPP Technical Requirement TR 45.912 and in the 3GPP Technical Specification TS 43.064.
The GPRS/EGPRS USF can be read by RTTI-capable MSs. The same principle discussed above of causing the MS to believe something else has been transmitted is used when RTTI radio blocks are sent in the downlink. The legacy (GPRS or EGPRS) MS is led to believe that legacy radio blocks have been transmitted. The USF bits are put in the positions where the legacy MS expects them to be, and the bits are encoded in the same way. A consequence of placing the USF bits in the positions where the legacy MS expects them to be is that two RTTI radio blocks during a 20-ms period must be sent using the same modulation. There is no modulation restriction for legacy radio blocks.
FIG. 2 illustrates the reservation of legacy stealing bits for a GMSK-modulated transmission. When GMSK-modulated RTTI blocks are sent in the downlink, stealing bits are put in the positions where the legacy GPRS/EGPRS MS expects them (i.e., in four consecutive TDMA frames comprising a radio block on a single timeslot). Setting these stealing bits to the CS-4 stealing bit code word leads the legacy GPRS MS to believe that a CS-4 block has been sent, and it leads the EGPRS MS to believe that an MCS-1 to MCS-4 block has been sent (in both cases interleaved over 20 ms on one timeslot). As shown in FIG. 2, the stealing bits of GPRS/EGPRS are placed in the two closest positions (one on each side) to the training sequence (TSC) of each burst (i.e., a total of eight bits over 20 ms). To indicate CS-4/MCS-1 to MCS-4, the bits are set to 00010110.
FIG. 3 illustrates the reservation of legacy USF bits for RTTI radio blocks. The USF bits must also be placed where the legacy MS expects them (i.e., as in a CS-4 and MCS-1 to MCS-4 radio block). As shown in FIG. 3, there are a total of twelve USF bits per legacy radio block—three in each burst. The correct positions are different in the four bursts in a legacy radio block. Table 1 below shows the positions of the twelve USF bits for a GMSK-modulated radio block.
TABLE 1Burst #Positions00, 50, 100134, 84, 98218, 68, 8232, 52, 66
When the legacy stealing bits and USF bits have been reserved for legacy purposes, there remain 116−2−3=111 bits per burst for the RTTI blocks. Thus, over 10 ms on two timeslots, there are a total of 444 bits. Note that the RLC/MAC header (excluding USF) and RLC data part of the RTTI block fit perfectly into these 444 bits without modification. With this solution, legacy MSs are able to decode the USF when RTTI blocks are sent in the downlink.
There is also a conventional solution enabling 8PSK-modulated RTTI radio blocks to be sent in the downlink. This solution is not applicable to the present invention, and will not be discussed further herein.
According to the state-of-the-art, RLC/MAC control messages are sent to RTTI MSs using CS-1 coding with reduced TTI (10 ms). In order for the MS to read the USF, the legacy SB and USF positions must be reserved.
There are a number of problems with the conventional solutions described above. First, the RTTI MS must be able to distinguish between two types of RTTI blocks—CS-1 (for the Packet Associated Control Channel, PACCH, and RLC/MAC control signaling) and MCS-1 to MCS-4 (for RLC data). The stealing bits, which are normally used for this purpose by legacy MSs as described above, cannot be used since they have been reserved for legacy purposes. In other words, the legacy stealing bits will continue to be used in the legacy manner to allow a legacy MS to be multiplexed on the same PDCHs where RTTI blocks are sent and will therefore only make sense if read according to BTTI burst sequencing (i.e. something not done by an MS with a DL RTTI TBF).
A state-of-the-art solution to this problem is described in the 3GPP technical document GP-060200, “Multiplexing of RTTI and legacy MS on the same PDCH”, TSG GERAN #32. This solution uses blind detection as described below.
1. The RTTI MS deinterleaves the bits in the bursts comprising an RTTI block. The interleaving is exactly the same for CS-1 as for MCS-1 to MCS-4.
2. The RTTI MS assumes that an MCS-1 to MCS-4 block has been sent, and decodes the RLC/MAC header.                a. If the RLC/MAC header is correctly decoded (e.g., CRC check passes), an MCS-1 to MCS-4 block has been received and the RTTI MS continues to decode the RLC data.        b. If the RLC/MAC header is incorrectly decoded (e.g., CRC check fails), the RTTI MS assumes that a CS-1 block has been received and decodes it accordingly.        
The problem with this state-of-the-art solution is that double decoding is necessary. This increases decoding complexity.
A second problem arises because the USF is encoded together with the data in a CS-1 block. Therefore, there are no available bits in the RTTI block to reserve for the stealing flags (SFs) or for the USF.
A state-of-the-art solution to this problem is described in the 3GPP technical document GP-070169, “Coding Scheme Update of RTTI”, TSG GERAN #33. In this solution, bits in the CS-1 block are punctured to give room for the SF/USF bits. The payload bits in the CS-1 block originally carried by the bits at these positions (see Table 1) are substituted and lost, so the downlink payload reception is adversely affected. The current coding schemes need to be updated to minimize this influence.
Thus the problem with this state-of-the-art solution is that bits are punctured. This degrades link performance of the RLC/MAC control messages, which are crucial for proper operation of the link.